View source for Messier Index/M104

{{bookify}}
{{Infobox galaxy 
| name = Sombrero Galaxy
| image = [[image:M104 ngc4594 sombrero galaxy hi-res.jpg|300px]]
| caption = The Sombrero Galaxy (M104) as observed by the Hubble Space Telescope (HST). Credit: HST/NASA/ESA.
| constellation name = Virgo<ref name="sinnott1988">{{cite book 
 | author=R. W. Sinnott, editor
 | title= The Complete New General Catalogue and Index Catalogue of Nebulae and Star Clusters by J. L. E. Dreyer
 | year=1988
 | publisher=Sky Publishing Corporation and Cambridge University Press
 | id={{ISBN|0-933-34651-4}}}}</ref>
| epoch = J2000
| ra = {{RA|12|39|59.4}}<ref name="ned">{{cite web
  | title=NASA/IPAC Extragalactic Database
  | work=Results for NGC 4594
  | url=http://nedwww.ipac.caltech.edu/
  | accessdate=2006-11-22}}</ref>
| dec = {{DEC|-11|37|23}}<ref name="ned" />
| dist_ly = 29.3 ± 1.6 Mly (9.0 ± 0.5 Mpc)<ref name="fordetal1996">{{cite journal
  | author=H. C. Ford, X. Hui, R. Ciardullo, G. H. Jacoby, K. C. Freeman
  | title=The Stellar Halo of M104. I. A Survey for Planetary Nebulae and the Planetary Nebula Luminosity Function Distance
  | journal=Astrophysical Journal
  | year=1996
  | volume=458
  | pages=455-466
  | url=http://adsabs.harvard.edu/abs/1996ApJ...458..455F  | doi=10.1086/176828
}}</ref><ref name="jensenetal2003">{{cite journal
  | author=Jensen, Joseph B.; Tonry, John L.; Barris, Brian J.; Thompson, Rodger I.; Liu, Michael C.; Rieke, Marcia J.; Ajhar, Edward A.; Blakeslee, John P.
  | title=Measuring Distances and Probing the Unresolved Stellar Populations of Galaxies Using Infrared Surface Brightness Fluctuations
  | journal=Astrophysical Journal
  | year=2003
  | date=February 2003
  | volume=583
  | issue=2
  | pages=712-726
  | url=http://adsabs.harvard.edu/abs/2003ApJ...583..712J
   | doi=10.1086/345430
}}</ref>{{Ref|A|a}}
| z = 1024 ± 5 km/s<ref name="ned" />
| type = SA(s)a<ref name="ned" />
| appmag_v = 9.0<ref name="ned" />
| size_v = 8&prime;.7 &times; 3&prime;.5<ref name="ned" />
| names = Messier 104,<ref name="ned" /> NGC 4594,<ref name="ned" /> PGC 42407,<ref name="ned" /> UGCA 293<ref name="ned" />
}}

The '''Sombrero Galaxy''' (also known as ''M104'' or ''NGC 4594'') is an unbarred spiral galaxy in the constellation Virgo.  It has a bright nucleus, an unusually large central bulge, and a prominent dust lane in its inclined disk.  The dark dust lane and the bulge give this galaxy the appearance of a sombrero.  The galaxy has an apparent magnitude of 
9.0, making it a galaxy that can easily be seen with amateur telescopes.  The large bulge, the central supermassive black hole, and the dust lane all attract the attention of professional astronomers.

==History==
===Discovery===
The Sombrero Galaxy was discovered in March of 1767 by Pierre Méchain, who described the object in a May 1767 letter to J. Bernoulli that was later published in the '''''Berliner Astronomisches Jahrbuch'''''.<ref name="kepplesanner1998">{{cite book 
 | author=G. R. Kepple, G. W. Sanner
 |title= The Night Sky Observer's Guide, Volume 2
 |publisher= Willmann-Bell, Inc.
 |year= 1998
 |id= {{ISBN|0-943396-60-3}}
 |pages=451
}}</ref><ref name="jones1991">{{cite book 
  | author=K. G. Jones
  | year=1991
  | title=Messier's Nebulae and Star Clusters
  | edition= 2nd edition
  | publisher=Cambridge University Press
  | location=Cambridge
  | id={{ISBN|0-521-37079-5}} }}</ref>  Charles Messier made a hand-written note about this and five other objects (now collectively recognized as M104 - M109) to his personal list of objects now known as the Messier Catalogue, but it was not "officially" included until 1921.<ref name="jones1991" />  William Herschel independently discovered the object in 1784 and additionally noted the presence of a "dark stratum" in the galaxy's disk, what is now called a dust lane.<ref name="kepplesanner1998" /><ref name="jones1991" />  Later astronomers were able to connect Méchain's and Herschel's observations.<ref name="jones1991" />

===Designation as a Messier object===
In 1921, Camille Flammarion found Messier's personal list of the Messier objects including the hand-written notes about the Sombrero Galaxy.  This was identified with object 4594 in the New General Catalogue, and Flammarion declared that it should be included in the Messier Catalogue.  Since this time, the Sombrero Galaxy has been known as ''M104''<ref name="jones1991" />.

===Historical redshift measurements===
In the 1910s, Vesto Slipher discovered that the spectra of several galaxies, including the Sombrero Galaxy, are redshifted.  The average velocity calculated from these redshifts was 400 km/s.  The redshift for the Sombrero Galaxy itself was calculated to be 1100 km/s<ref name="slipher1915">{{cite journal
  | author=V. M. Slipher
  | title=Spectrographic Observations of Nebulae
  | journal=Popular Astronomy
  | year=1915
  | volume=23
  | pages=21-24
  | url=http://adsabs.harvard.edu/abs/1915PA.....23...21S}}</ref>.  Slipher's spectra were among the first observations of the expansion of the universe, one of the key pieces of evidence for the Big Bang Theory.

Slipher also detected rotation within the spectra of the Sombrero Galaxy.  His observations of galaxy rotation are among the first ever performed<ref name="slipher1915" />.

[[image:M104 3.6 4.5 8.0 microns spitzer.png|thumb|left|300px|A mid-infrared image of the Sombrero Galaxy as seen by the Spitzer Space Telescope (SST).  The white colors represent 3.6 and 5.8 [[w:micrometre emission|micrometre emission]] from starlight.  The red colors represent 8.0 micrometre emission from polycyclic aromatic hydrocarbons (PAHs) within the dust ring.<ref name="bendoetal2006">{{cite journal
  | author= G. J. Bendo, B. A. Buckalew, D. A. Dale, B. T. Draine, R. D. Joseph, R. C. Kennicutt Jr., K. Sheth, J.-D. T. Smith, F. Walter, D. Calzetti, J. M. Cannon, C. W. Engelbracht, K. D. Gordon, G. Helou, D. Hollenbach, E. J. Murphy, H. Roussel
  | title=Spitzer and JCMT Observations of the Active Galactic Nucleus in the Sombrero Galaxy (NGC 4594)
  | journal=Astrophysical Journal
  | year=2006
  | volume=645
  | pages=134-147
  | url=http://adsabs.harvard.edu/abs/2006ApJ...645..134B   | doi= 10.1086/504033
}}</ref>  Credit: Spitzer Infrared Nearby Galaxies Survey/SST/NASA.]]

==Dust ring==
As noted above, this galaxy's most striking feature is the dust lane that crosses in front of the bulge of the galaxy.  This dust lane is actually a symmetric ring that encloses the bulge of the galaxy.<ref name="bendoetal2006" />  Most of the cold atomic hydrogen gas<ref name="bajajaetal1984">{{cite journal
  | author= E. Bajaja, G. van der Burg, S. M.; Faber, J. S. Gallagher, G. R. Knapp, W. W. Shane
  | title=The distribution of neutral hydrogen in the Sombrero galaxy, NGC 4594
  | journal=Astronomy and Astrophysics
  | year=1984
  | volume=141
  | pages=309-317
  | url=http://adsabs.harvard.edu/abs/1984A&A...141..309B }}</ref> and the dust<ref name="bendoetal2006" /> lies within this ring.  The ring might also contain most of the Sombrero Galaxy's cold molecular gas,<ref name="bendoetal2006" /> although this is an inference based on observations with low resolution and weak detections.<ref name="bajajaetal1988">{{cite journal
  | author= E. Bajaja, E. Hummel, R. Wielebinski, R.-J. Dettmar
  | title=The large-scale radio continuum structure of the Sombrero galaxy (NGC 4594)
  | journal=Astronomy and Astrophysics
  | year=1988
  | volume=202
  | pages=35-40
  | url=http://adsabs.harvard.edu/abs/1988A&A...202...35B }}</ref><ref name="youngetal1995">{{cite journal
  | author= J. S. Young, S. Xie, L. Tacconi, P. Knezek, P. Viscuso, L. Tacconi-Garman, N. Scoville, S. Schneider, F. P. Schloerb, S. Lord, A. Lesser, J. Kenney, Y.-L. Huang, N. Devereux, M. Claussen, J. Case, J. Carpenter, M. Berry, L. Allen
  | title=The FCRAO Extragalactic CO Survey. I. The Data
  | journal=Astrophysical Journal Supplement
  | year=1995
  | volume=98
  | pages=219-257
  | url=http://adsabs.harvard.edu/abs/1995ApJS...98..219Y   | doi= 10.1086/192159
}}</ref>  Additional observations are needed to confirm that the Sombrero galaxy's molecular gas is constrained to the ring.  Based on infrared spectroscopy, the dust ring is the primary site of star formation within this galaxy.<ref name="bendoetal2006" />

==Nucleus==
The nucleus of the Sombrero galaxy is classified as a low ionization nuclear emission region|low ionization nuclear emission region (LINER).<ref name="hoetal1997">{{cite journal
  | author= L. C. Ho, A. V. Filippenko, W. L. W. Sargent
  | title=A Search for "Dwarf" Seyfert Nuclei. III. Spectroscopic Parameters and Properties of the Host Galaxies
  | journal=Astrophysical Journal Supplement
  | year=1997
  | volume=112
  | pages=315-390
  | url=http://adsabs.harvard.edu/abs/1997ApJS..112..315H   | doi= 10.1086/313041
}}</ref>  These are nuclear regions where ionized gas is present, but the ions are only weakly ionized (i.e. the atoms are missing relatively few electrons).  The source of energy for ionizing the gas in LINERs has been debated extensively.  Some LINER nuclei may be powered by hot, young stars found in star formation regions, whereas other LINER nuclei may be powered by active galactic nuclei (highly energetic regions that contain supermassive black holes).  Infrared spectroscopy observations have demonstrated that the nucleus of the Sombrero Galaxy is probably devoid of any significant star formation activity.  However, a supermassive black hole has been identified in the nucleus (as discussed in the subsection below), so this active galactic nucleus is probably the energy source that weakly ionizes the gas in the Sombrero Galaxy.<ref name="bendoetal2006" />

===Central supermassive black hole===
In the 1990s, a research group led by John Kormendy demonstrated that a supermassive black hole is present within the Sombrero Galaxy.<ref name="kormendyetal1996">{{cite journal
  | author=J. Kormendy, R. Bender, E. A. Ajhar, A. Dressler, S. M. Faber, K. Gebhardt, C. Grillmair, T. R. Lauer, D. Richstone, S. Tremaine
  | title=Hubble Space Telescope Spectroscopic Evidence for a 1 X 10 9 M☉ Black Hole in NGC 4594
  | journal=Astrophysical Journal Letters
  | year=1996
  | volume=473
  | pages=L91-L94
  | url=http://adsabs.harvard.edu/abs/1996ApJ...473L..91K }}</ref>  
Using spectroscopy data from both the CFHT
and the Hubble Space Telescope, the group showed that the speed of rotation of the stars within the center of the galaxy could not be maintained unless a mass 1 billion times the mass of the Sun, or 10<sup>9</sup>M<sub>☉</sub>, is present in the center.<ref name="kormendyetal1996" />  This is among the most massive black holes measured in any nearby galaxies.

===Synchrotron emission===
At radio and X-ray wavelengths, the nucleus is a strong source of synchrotron emission.<ref name="debruynetal1996">{{cite journal
  | author=A. G. de Bruyn, P. C. Crane, R. M. Price, J. B. Carlson
  | title=The radio sources in the nuclei of NGC 3031 and NGC 4594
  | journal=Astronomy and Astrophysics
  | year=1976
  | volume=46
  | pages=243-251
  | url=http://adsabs.harvard.edu/abs/1976A&A....46..243D }}</ref><ref name="hummeletal1984">{{cite journal
  | author=E. Hummel, J. M. van der Hulst, J. M. Dickey
  | title=Central radio sources in spiral galaxies - Starburst or accretion
  | journal=Astronomy and Astrophysics
  | year=1984
  | volume=134
  | pages=207-221
  | url=http://adsabs.harvard.edu/abs/1984A&A...134..207H }}</ref><ref name="theanetal2000">{{cite journal
  | author=A. Thean, A. Pedlar, M. J. Kukula, S. A. Baum, C. P. O'Dea
  | title=High-resolution radio observations of Seyfert galaxies in the extended 12-μm sample - I. The observations
  | journal=Monthly Notices of the Royal Astronomical Society
  | year=2000
  | volume=314
  | pages=573-588
  | url=http://adsabs.harvard.edu/abs/2000MNRAS.314..573T   | doi=10.1046/j.1365-8711.2000.03401.x
}}</ref><ref name="dimatteoetal2001">{{cite journal
  | author=T. Di Matteo, C. L. Carilli, A. C. Fabian
  | title=Limits on the Accretion Rates onto Massive Black Holes in Nearby Galaxies
  | journal=Astrophysical Journal
  | year=2001
  | volume=547
  | pages=731-739
  | url=http://adsabs.harvard.edu/abs/2001ApJ...547..731D }}</ref><ref name="pellegrinietal2002">{{cite journal
  | author=S. Pellegrini, G. Fabbiano, F. Fiore, G. Trinchieri, A. Antonelli
  | title=Nuclear and global X-ray properties of LINER galaxies: Chandra and BeppoSAX results for Sombrero and NGC 4736
  | journal=Astronomy and Astrophysics
  | year=2002
  | volume=383
  | pages=1-13
  | url=http://adsabs.harvard.edu/abs/2002A&A...383....1P   | doi=10.1051/0004-6361:20011482
}}</ref><ref name="pellegrinietal2003">{{cite journal
  | author=S. Pellegrini, A. Baldi, G. Fabbiano, D.-W. Kim
  | title=An XMM-Newton and Chandra Investigation of the Nuclear Accretion in the Sombrero Galaxy (NGC 4594)
  | journal=Astrophysical Journal
  | year=2003
  | volume=597
  | pages=175-185
  | url=http://adsabs.harvard.edu/abs/2003ApJ...597..175P   | doi=10.1086/378235
}}</ref><ref name="krauseetal2006">{{cite journal
  | author=M. Krause, R. Wielebinski, M. Dumke
  | title=Radio polarization and sub-millimeter observations of the Sombrero galaxy (NGC 4594). Large-scale magnetic field configuration and dust emission
  | journal=Astronomy and Astrophysics
  | year=2006
  | volume=448
  | pages=133-142
  | url=http://adsabs.harvard.edu/abs/2006A&A...448..133K }}</ref>  Synchrotron emission is produced when high velocity electrons oscillate as they pass through regions with strong [[w:magnetic fields]].  This emission is actually quite common for [[w:active galactic nuclei]].  Although radio synchrotron emission may vary over time for some active galactic nuclei, the luminosity of the radio emission from the Sombrero Galaxy only varies 10-20%.<ref name="debruynetal1996"/>

===Unidentified submillimeter emission===
In 2006, two groups published measurements of the submillimeter radiation from the nucleus of the Sombrero Galaxy at a wavelength of 850 [[micrometres.<ref name="krauseetal2006" /><ref name="bendoetal2006" />  This submillimeter emission was found not to originate from the thermal emission from dust (which is commonly seen at infrared and submillimeter wavelengths), synchrotron emission (which is commonly seen at radio wavelengths), bremsstrahlung emission from hot gas (which is uncommonly seen at millimeter wavelengths), or molecular gas (which commonly produces submillimeter spectral lines).<ref name="bendoetal2006" />  The source of the submillimeter emission remains unidentified.

==Globular clusters==
The Sombrero Galaxy has a relatively large number of globular clusters.  Observational studies of globular clusters in the Sombrero Galaxy have produced estimates of the population in the range of 1200 to 2000.<ref name="wakamatsu1977">{{cite journal
  | author=K.-I. Wakamatsu
  | title=Radial distribution and total number of globular clusters in M104
  | journal=Publications of the Astronomical Society of the Pacific
  | year=1977
  | volume=89
  | pages=267-270
  | url=http://adsabs.harvard.edu/abs/1977PASP...89..267W}}</ref><ref name="harrisetal1984">{{cite journal
  | author=W. E. Harris, H. C. Harris, G. L. H. Harris
  | title=Globular clusters in galaxies beyond the local group. III NGC 4594 (the Sombrero)
  | journal=Astronomical Journal
  | year=1984
  | volume=89
  | pages=216-223
  | url=http://adsabs.harvard.edu/abs/1984AJ.....89..216H  | doi=10.1086/113504
}}</ref><ref name="bridgeshaynes1992">{{cite journal
  | author=T. J. Bridges, D. A. Hanes
  | title=The globular cluster system of NGC 4594 (the Sombrero)
  | journal=Astronomical Journal
  | year=1992
  | volume=103
  | pages=800-814
  | url=http://adsabs.harvard.edu/abs/1992AJ....103..800B  | doi=10.1086/116102
}}</ref>  The ratio of the number of globular clusters to the total luminosity of the galaxy is high compared to the Milky Way and similar galaxies with small bulges, but the ratio is comparable to other galaxies with large bulges.  These results have been repeatedly used to demonstrate that the 
number of globular clusters in galaxies is thought to be related to the size of the galaxies' bulges.  The surface density of the globular clusters generally follows the light profile of the bulge except for near the center of the galaxy.<ref name="wakamatsu1977" /><ref name="bridgeshaynes1992" /><ref name="">{{cite journal
  | author=S. S. Larsen, D. A. Forbes, J. P. Brodie
  | title=Hubble Space Telescope photometry of globular clusters in the Sombrero galaxy
  | journal=Monthly Notices of the Royal Astronomical Society
  | year=2001
  | volume=327
  | pages=1116-1126
  | url=http://adsabs.harvard.edu/abs/2001MNRAS.327.1116L  | doi=10.1046/j.1365-8711.2001.04797.x
}}</ref>

==Distance==
At least two methods have been used to measure the distance to the Sombrero Galaxy.  

The first method relies on comparing the measured fluxes from planetary nebulae in the Sombrero Galaxy to the known luminosities of planetary nebulae in the Milky Way.  This method gave the distance to the Sombrero Galaxy as 29.0 ± 2.0 Mly (8.9 ± 0.6 Mpc).<ref name="fordetal1996"/>  

The other method used is the surface brightness fluctuations method.  This method uses the grainy appearance of the galaxy's bulge to estimate the distance to it.  Nearby galaxy bulges will appear very grainy, while more distant  bulges will appear smooth.  Early measurements using this technique gave distances of 30.6 ± 1.3 Mly (9.4 ± 0.4 Mpc).<ref name="ajharetal1997">{{cite journal
  | author=E. A. Ajhar, T. R. Lauer, J. L. Tonry, J. P. Blakeslee, A. Dressler, J. A. Holtzman, M. Postman
  | title=Calibration of the Surface Brightness Fluctution Method for use with the Hubble Space Telescope
  | journal=Astronomical Journal
  | year=1997
  | volume=114
  | pages=626-634
  | url=http://adsabs.harvard.edu/abs/1997AJ....114..626A  | doi=10.1086/118498
}}</ref>  Later, after some refinement of the technique, a distance of 32 ± 3 Mly (9.8 ± 0.8 Mpc) was measured.<ref name="tonryetal2001">{{cite journal
  | author=J. L. Tonry, A. Dressler, J. P. Blakeslee, E. A. Ajhar, A. B. Fletcher, G. A. Luppino, M. R. Metzger, C. B. Moore
  | title=The SBF Survey of Galaxy Distances. IV. SBF Magnitudes, Colors, and Distances
  | journal=Astrophysical Journal
  | year=2001
  | volume=546
  | issue=2
  | pages=681-693
  | url=http://adsabs.harvard.edu/abs/2001ApJ...546..681T  | doi=10.1086/318301
}}</ref>  This was even further refined in 2003 to be 29.6 ± 2.5 Mly (9.1 ± 0.8 Mpc).<ref name="jensenetal2003" />

The average distance measured through these two techniques is 29.3 Mly (9.0 Mpc) with an uncertainty of 1.6 Mly (0.5 Mpc).{{Ref|B|b}}

==Nearby galaxies and galaxy group information==
The Sombrero Galaxy lies within a complex, filament-like cloud of galaxies that extends to the south of the Virgo Cluster.<ref name="nbg">{{cite book 
  | author=R. B. Tully
  | year=1988
  | title=Nearby Galaxies Catalog
  | publisher=Cambridge University Press
  | location=Cambridge
  | id={{ISBN|0-521-35299-1}}}}</ref>  However, it is unclear as to whether the Sombrero Galaxy is part of a formal galaxy group.  Hierarchical methods for identifying groups, which determine group membership by considering whether individual galaxies belong to a larger aggregate of galaxies, typically produce results showing that the Sombrero Galaxy is part of a group that includes NGC 4487, NGC 4504, NGC 4802, UGCA 289, and possibly a few other galaxies.<ref name="nbg" /><ref name="fouqueetal1992">{{cite journal
  | author=P. Fouque, E. Gourgoulhon, P. Chamaraux, G. Paturel
  | title=Groups of galaxies within 80 Mpc. II - The catalogue of groups and group members
  | journal=Astronomy and Astrophysics Supplement
  | year=1992
  | volume=93
  | pages=211-233
  | url=http://adsabs.harvard.edu/abs/1992A&AS...93..211F}}</ref><ref name="giuricinetal2002">{{cite journal
  | author=G. Giuricin, C. Marinoni, L. Ceriani, A. Pisani
  | title=Nearby Optical Galaxies: Selection of the Sample and Identification of Groups
  | journal=Astrophysical Journal
  | year=2000
  | volume=543
  | pages=178-194
  | url=http://adsabs.harvard.edu/abs/2000ApJ...543..178G  | doi=10.1086/317070
}}</ref>  However, results that rely on the percolation method (i.e. the "friends-of-friends" method), which links individual galaxies together to determine group membership, indicate that either the Sombrero Galaxy is not in a group<ref name="garcia1993">{{cite journal
  | author=A. Garcia
  | title=General study of group membership. II - Determination of nearby groups
  | journal=Astronomy and Astrophysics Supplement
  | year=1993
  | volume=100
  | pages=47-90
  | url=http://adsabs.harvard.edu/abs/1993A&AS..100...47G}}</ref> or that it may only be part of a galaxy pair with UGCA 287.<ref name="giuricinetal2002" />

==Amateur astronomy observation information==
The Sombrero Galaxy is located 11.5° west of Spica<ref name="jones1991"/> and 5.5° northeast of Eta Corvi.<ref name="omeara1998">{{cite book 
  | author=S. J. O'Meara
  | year=1998
  | title=The Messier Objects
  | edition= 
  | publisher=Cambridge University
  | location=Cambridge
  | id={{ISBN|0-521-55332-6}}}}</ref>  Although the galaxy is visible with 7x35 binoculars or a 4 inch amateur telescope,<ref name="omeara1998"/> an 8 inch telescope is needed to distinguish the bulge from the disk,<ref name="jones1991"/> and a 10 or 12 inch telescope is needed to see the dark dust lane.<ref name="jones1991"/>

==External links==
* [http://www.spacetelescope.org/images/html/opo0328a.html Hubble Mosaic of the Majestic Sombrero Galaxy (ESA)]
* [http://hubblesite.org/newscenter/newsdesk/archive/releases/2003/28/image/a HubbleSite: The Majestic Sombrero Galaxy (M104)]
* [http://www.nightskyinfo.com/archive/m104_galaxy/ M104, the Sombrero Galaxy (NightSkyInfo.com)]
* [http://www.seds.org/messier/m/m104.html Messier 104 (SEDS)]
* [http://chandra.harvard.edu/photo/2007/sombrero/ Sombrero Galaxy:A Great Observatories View]
* [http://www.dsi-astronomie.de/M104.htm M104 The Sombrero Galaxy]

==Notes==
<div class="references-small">
:{{Note|A|}}{{Note|B|}}average(29.6 ± 2.5, 29.0 ± 2) = ((29.6 + 29.0) / 2) ± ((2.5<sup>2</sup> + 2.0<sup>2</sup>)<sup>0.5</sup> / 2) = 29.3 ± 1.6
</div>

==References==
{{reflist|2}}

{{Messier Index}}